The invention relates to the air conditioning of the passenger compartment of motor vehicles.
For this function, recourse is usually made to a closed loop air-conditioning system having a loop of refrigerant fluid and including a compressor, a condenser, a pressure-reduction valve and an evaporator, the latter being in contact with an airflow to be cooled.
In early systems, the compressor was continuously driven by the engine of the vehicle so as to produce a throughput of refrigerant fluid determined by the speed of the engine. The airflow, greatly cooled by the evaporator, then passed through the radiator for heating the passenger compartment so as to be adjusted to the desired temperature.
The cooling of the air, followed by reheating, was hardly satisfactory in terms of energy economy. To remedy that, recourse was made to variable-displacement compressors, called internal-control compressors, the throughput of which could be set independently of their speed of rotation within the range of regulation of the displacement, by virtue of a valve the position of which was such that the entry pressure of the fluid was a function of the exit pressure. As the settings obtained were still not sufficiently fine externally controlled, variable-displacement compressors were developed. These compressors have a solenoid valve, preferably a chopper-type valve, the power-supply current of which determines the entry pressure of the fluid into the compressor. With this pressure being equal, to within a loss of pressure head, to that prevailing in the evaporator, which determines the operating temperature of the evaporator, it is thus possible to set the effectiveness of the air-conditioning loop with precision. An example of an external-control, variable-displacement compressor is described in EP-A-0 353 764.
In the event of leakage in the loop, the quantity of refrigerant fluid contained within the system will diminish. Below a refrigerant-fluid level threshold, the compressor risks being damaged in operation.
Moreover, it can happen that the control valve of the compressor becomes blocked and no longer allows the cooling power produced to be adjusted, whatever the power-supply current which is applied to it.
The object of the invention is to make it possible to detect operating anomalies in a closed loop air-conditioning system comprising a variable-throughput compressor, and especially insufficient refrigerant-fluid level or a blockage of the control valve of the compressor.
The invention envisages, in particular, a method for checking the state of a closed loop system for air conditioning the passenger compartment, of a motor vehicle, especially a system comprising an evaporator suitable for cooling an airflow to be sent into the passenger compartment, and a variable-throughput compressor controlled by an electrical control signal, in which method the temperature of the said airflow is determined by means of a sensor, a corresponding theoretical value of the said temperature is defined, for at least one value of the said control signal, the actual value of the temperature is compared with the theoretical value and the result of this comparison is used as an indicator of the normal or abnormal state of the loop.
The result of the comparison may especially indicate a low level of refrigerant fluid in the loop.
This is because, in normal operation, the major part of the volume of the evaporator is full of refrigerant fluid in liquid/gas equilibrium, the temperature of which is determined by the pressure at the entry to the compressor, as indicated above. If the quantity of refrigerant fluid in the closed loop system is insufficient, then the evaporator contains only a small quantity of fluid in the liquid state, or even none at all, and on the contrary is filled entirely, or almost entirely, with fluid in the superheated gaseous state, that is to say at a temperature higher than the liquid/gas equilibrium temperature. A temperature sensor in contact with the evaporator for detecting the temperature of the airflow, or in contact with the air immediately downstream thereof, placed facing an area of the evaporator where the fluid is normally in liquid/gas equilibrium, will then be facing a superheated area and will detect a temperature higher than the theoretical temperature.
In the case of an external-control variable-displacement compressor as set out above, blockage of the valve of the compressor may also be revealed by the result of the comparison.
The choice of the power-supply current for the valve then no longer has any effect on the pressure at the entry to the compressor and consequently on the temperature of the airflow, the latter being just as likely to be above as below the theoretical value.
Optional characteristics of the invention, which are complementary or alternative, are set out below:
any low level of the refrigerant fluid is indicated by the fact that the actual temperature of the airflow exceeds a predetermined threshold with respect to the theoretical temperature.
in order to check the level of refrigerant fluid, the comparison is carried out after having applied a control current for sufficient time to produce a high throughput of the fluid.
any blockage of the valve of the compressor is indicated by the fact that the actual temperature of the airflow remains constant when the control current, and consequently the theoretical temperature, is varied.
the compressor is driven electrically.
the theoretical value or the threshold is defined by using parameters, in combination with the control signal, which link the temperature of the airflow to the pressure at the entry to the compressor.
the parameters comprise at least one of the following parameters: state of a blower producing the said airflow, position of a flap for recycling the air of the passenger compartment, speed of the vehicle, temperature of the air outside and inside the passenger compartment, relative humidity and temperature of the incident airflow and throughput of refrigerant fluid in the loop.
The sensor may be placed in the vicinity either of the superheated area, or of the coldest point of the evaporator, or else at a point of the evaporator taken to an intermediate temperature.